1. Field of the Invention
This invention relates to a color display system in electrophoretic display devices.
2. Related Background Art
In recent years, with advancement of information machinery, the quantity of data of various information is becoming larger and larger, and the information is also outputted in various forms. The outputting of information is commonly roughly grouped into “display representation” making use of a cathode-ray tube, a liquid-crystal display panel or the like and “hard-copy representation” on paper by means of a printer or the like.
In the display representation, there is an increasing need for display devices of low power consumption and small thickness. In particular, liquid-crystal display devices have energetically been developed and commercialized as display devices adapted for such need. In liquid-crystal display devices available at present, however, characters or letters displayed on a screen may be viewed with difficulty depending on the angles at which the screen is viewed or under the influence of reflected light, and the load on the eyes which is caused by flickering, low luminance and so forth of a light source has not been properly solved. Also, in the display representation making use of a cathode-ray tube, although it provides sufficient contrast and luminance compared with the liquid-crystal display, it may cause flickering, and can not be said to have a sufficient display quality level compared with the hard-copy representation discussed below. In addition, its display units are so large and heavy as to have a very low portability.
Meanwhile, the hard-copy representation has been considered to become unnecessary as information is made electronic, but in fact hard copies are still taken in vast quantities. As reasons therefor, the following can be given. In the case of display representation of information in addition to the above problem concerning the display quality level, the display has a resolution of 120 dpi at maximum, which is fairly lower than that of prints on paper (usually 300 dpi or higher). Hence, the display representation may greatly task the eyes compared with the hard-copy representation. As a result, even though the information can be viewed on a display, it is first outputted on a hard copy. In addition, the information represented on hard copies can be arranged in a large number of sheets without any limitation of the display area as in the display representation, can be rearranged without any complicated machine operation, or can be checked in order. These are also large reasons why the hard-copy representation is used in combination even though the display representation is feasible. Furthermore, the hard-copy representation does not require any energy for retaining its representation, and has a superior portability so that the information can be checked anytime and anywhere as long as the information is not so extremely much.
Thus, as long as any motion-picture display or frequent rewriting is not required, the hard-copy representation has various advantages different from the display representation, but has such a disadvantage that paper is consumed in a large quantity. Accordingly, in recent years, development is energetically put forward on a rewritable recording medium (a recording medium on which highly visible images can repeatedly be recorded and erased in many cycles and which does not require any energy for retaining its representation). The third way of representation which has succeeded the features the hard copies have and in which images are rewritable is herein refered to as “paper-like display”.
Requirements for the paper-like display are such that images are rewritable, that energy for retaining the display is not required or is sufficiently low (memory performance), that the display has a good portability, that the display has a good quality level, and so forth. At present, as a representation method which can be regarded as the paper-like display, for example, a reversible display medium is available using an organic low-molecular and high-molecular resin matrix system which is recorded and erased with a thermal printer head (e.g., Japanese Patent Applications Laid-Open No. S55-154198 and No. S57-82086). This system is partly utilized as a display area of a prepaid card, but has problems such that the contrast is not so high and the writing and erasing can only be repeated as small as 150 to 500 times.
As a way of display which is expected to be utilized as another paper-like display, an electrophoretic display device invented by Harold D. Lees et al. (U.S. Pat. No. 3,612,758) is known. Besides, Japanese Patent Application Laid-Open No. H9-185087 discloses an electrophoretic display device.
This display device is constituted of a dispersion medium having an insulating liquid in which colored charged particles stand dispersed, and a pair of electrodes which are set face to face holding this dispersion medium between them. It is a device in which, upon application of a voltage to the dispersion medium via the electrodes, the colored charged particles are attracted by Coulomb force to the electrode side having a polarity reverse to that of electric charges the particles themselves have, by utilizing electrophoretic properties of the colored charged particles. Its display is performed utilizing differences between the color of the colored charged particles and the color of an insulating liquid having been dyed. That is, the color of the colored charged particles is perceived when the colored charged particles are kept attracted to the surface of a first electrode near to the observer side and having light transmission properties. On the contrary, when the colored charged particles are kept attracted to the surface of a second electrode distant from the observer side, the color of the insulating liquid having been dyed is perceived, which has been so dyed as to have optical characteristics different from those of the colored charged particles.
However, in such an electrophoretic display device (hereinafter often refered to as “vertical-movement type electrophoretic display device”), a coloring material such as a dye or ions must be mixed in the insulating liquid, and the presence of such a coloring material tends to act as an unstable factor in electrophoretic movement because it brings about the delivering and receiving of additional electric charges, resulting in a lowering of performance, lifetime and stability as a display device in some cases.
In order to solve such a problem, an electrophoretic display device in which a pair of electrodes consisting of a first display electrode and a second display electrode are disposed on the same substrate and the charged particles are moved horizontally as viewed from the observer side, has been proposed as disclosed in Japanese Patent Applications Laid-Open No. S49-5598 and No. H10-005727. It is a device in which, utilizing electrophoretic properties of colored charged particles, display is performed by making the colored charged particles move horizontally to the substrate surface between the surface of the first display electrode and the surface of the second display electrode in a transparent insulating liquid by applying a voltage.
In such a horizontal-movement type electrophoretic display device, the insulating liquid is transparent in many cases. As viewed from the observer side, the first display electrode and the second display electrode are differently colored, and either of their colors has been made to have the same color as the colored charged particles. For example, where the color of the first display electrode is black, the color of the second display electrode is white and the color of the colored charged particles is black, the second display electrode comes uncovered to see white when the colored charged particles stand distributed over the first display electrode, and see black as the color of the colored charged particles when the colored charged particles stand distributed over the second display electrode.
Now, the most fundamental system for materializing color display in the above electrophoretic display devices is a system in which three unit cells respectively having the three primary colors consisting of RGB or YMC are disposed in parallel on the same plane to make up each pixel and the color display is performed by the principle of additive color mixing. In either system of the vertical-movement type and the horizontal-movement type, each unit cell has one type of colored charged particles, two drive electrodes and a colored migration liquid, where two colors, the color of the colored charged particles and the color of the colored migration liquid, or the color of the colored charged particles and the color of a color filter, can be shown by the movement of the particles.
For example, in Japanese Patent Applications Laid-Open No. 2000-035589, three unit cells having different colored liquids with the three primary colors are disposed in parallel to form each pixel. Unit cells formed of microcapsules in which a colored liquid and white particles have been enclosed are ejected from nozzles so that microcapsules having different colored liquids (migration liquids) with the three primary colors, yellow (Y), cyan (C) and magenta (M) are regularly arranged. Each microcapsule changes alternately in two colors, the white which is the color of the particles and the color of the migration liquid, by the vertical movement of the white particles.
Also in the case of the horizontal-movement type, three unit cells showing different colors for color display are arranged to make up each pixel. Each unit cell is filled with a transparent insulating liquid and black particles. On the display electrode surfaces of the unit cells, different color filters with the three primary colors, red (R), green (G) and blue (B), are respectively disposed in order from the left cell. Each unit cell changes alternately in two colors, the black which is the color of the particles and the color of each color filter, by the horizontal movement of the black particles.
In any of the above systems, when color display is performed, each pixel is formed by the three unit cells disposed adjoiningly and having colors corresponding to the three primary colors, and the desired display color is formed by the principle of additive color mixing.
However, in the additive color mixing of the three primary colors, it is theoretically impossible to achieve brightness and color sharpness (inclusive of sufficient black display) simultaneously, and it is very difficult to materialize a reflection type display device having the display quality level the printed mediums can have. In the case of the additive color mixing by the use of white particles plus the three primary colors Y, C and M, a satisfactory level can be achieved in respect of the brightness, but colors of pastel shades lacking in color sharpness are shown because the white light component is always superimposed on the background of reflected light, and also any sufficient black is not obtainable. A sufficient black is obtainable if black particles are used, but such a measure is insufficient in respect of the brightness and the color sharpness. On the other hand, in the case of the additive color mixing by the use of black particles plus the three primary colors R, G and B, the intensity ratio of reflected light to incident light is 1/9 or less in the monochrome display and ⅓or less in the white display, where any sufficient brightness is not achieved. The brightness is improved if white particles are used, but any sharp color representation is not obtainable and also any sufficient black is not obtainable.
In WO 99/53373, in order to improve brightness and sharpness of colors in the additive color mixing, a structure is disclosed in which unit cell microcapsules change in three colors. Three unit cells showing different colors for color display are arranged to make up each pixel. In this structure, which is called “dual particle curtain mode”, the microcapsules are filled with an migration liquid in which two types of colored charged particles having different charge polarities and colors have been dispersed. By applying voltage to three drive electrodes disposed in the unit cells, the two types of colored charged particles are moved independently, where each unit cell can be changed alternately in three colors, the three colors of the two types of colored charged particles and the color of the migration liquid, or the three colors of the two types of colored charged particles and the color of each color filter disposed on the back of the microcapsules. A typical cell structure disclosed in this WO 99/53373 is shown in FIGS. 17 to 18D. A display electrode is disposed on the side of a front substrate, and two collection electrodes to which different voltages can be applied are disposed on the side of a back substrate. Its insulating liquid is transparent, and a color filter is disposed on the back of each unit cell. For example, in the case of a combination of white charged particles with the electrophretic particles and a color filter standing in a complementary color relation to the charged particles, the sharpness and brightness of colors can be improved in respect of monochrome display (FIG. 18B) and complementary-color display (FIG. 18C).
In the structure shown in FIGS. 17 to 18D, however, the collection electrodes are both formed on the back substrate side, and hence, the open-area ratio, which is of the area in which the color filter disposed on the back of each unit cell functions effectively (the display area shown in FIG. 17), is remarkably reduced inevitably. As a result, there is a problem in that the sharpness and brightness of colors that are originally to be aimed at are insufficient.
Use of not the color filter but the colored insulating liquid enables avoidance of the above problem to a certain extent. In such a case, however, there is a problem in that a coloring dye added to the insulating liquid tends to cause deterioration in extensive operation due to electrode reaction and contrast deterioration due to the dyeing of charged particles.
There also is a problem in that since the number of electrodes become larger by one from the conventional two electrodes to the three electrodes, this consequently makes it necessary to narrow wiring pitches at a display medium panel portion or causes a rise in drive IC costs.